1. Field of the Invention
The present invention relates to a charging device for charging an image carrier, a process cartridge incorporating the charging device, and an image forming apparatus incorporating the charging device.
2. Description of the Related Art
One type of image forming apparatus that charges an image carrier with a charging device, forms an electrostatic latent image on the charged image carrier and then visualizes the electrostatic latent image as a toner image is well known in the prior art. Such an image forming apparatus may constitute an electronic copy machine, a facsimile machine, a printer, or a composite machine that has at least two functions mentioned above. A well-known charging device used in such an image forming apparatus is described in, for example, Japanese Patent Application Laid Open No. Hei 5-27555, including a charging member that is disposed opposite to an image carrier and applied with a voltage for charging the latter. The charging operation is effected by a discharge between the charging member and the image carrier. A charging member used in such a charging device is disposed opposite to the image carrier, and generally comprises a conductive substrate and a resistor layer thereon. The end areas of the resistor layer are disposed with spacers contacting with the image carrier, so that a small gap is created between the image carrier and the resistor layer between the spacers. Since the portion of the resistor layer opposite to the image carrier is located apart from the surface of the image carrier with a small gap between them, it is possible to reduce contamination on the surface of the resistor layer that is conventionally caused by contact between the image carrier and said portion of the resistor layer. Meanwhile, the aforementioned charging device is advantageous over those using Corona dischargers because the former generate less ozone and allow the use of a lower voltage applied to the charging member.
In the charging device used in the prior art, the end areas of the resistor layer are adhered with gap tapes by means of an adhesive. The gap tapes contacting with the image carrier so that a small gap is created between the surface of the image carrier and the resistor layer between the gap tapes. Though the gap tapes can serve as spacers, it is noted that the use of gap tapes causes the following problems.
The width of the small gap between the surface of the image carrier and the resistor layer between the spacers is preferably held constant for a long time. For example, as the width of the small gap continuously decreases, the resistor layer between the spacers will come into contact with the surface of the image carrier. Therefore, some contaminants like toners are adhered to the resistor layer, and the charging member is contaminated. Consequently, the charging characteristics for the image carrier are altered to cause abnormal electrification, and the quality of the toner image formed on the image carrier is deteriorated. On the contrary, as the width of the small gap continuously increases, abnormal discharge will occur to cause poor electrification of the image carrier, and the quality of the toner image therefore is deteriorated.
Moreover, the gap tapes are easily peeled off and damaged as subjected to an external force because they are merely adhered on the surface of the resistor layer. As the gap tapes are damaged like this, the width of the small gap between the surface of the image carrier and the resistor layer between the gap tapes is changed, and abnormal electrification of the image carrier is caused deteriorating the image quality.
Furthermore, since the gap tapes adhered to the surface of the resistor layer of the charging member with an adhesive are pressed on the image carrier and the image carrier comprises a hard material, a large pressure is imposed on the adhesive. Therefore, the adhesive is easily extruded from between the resistor layer and the gap tapes. Consequently, a small portion of the extreme small amount of toner on the surface of the image carrier is adhered to the extruded adhesive, and the thickness of the toner adhered to the adhesive increases time by time. As the thickness is eventually larger than the total thickness of the gap tapes and the adhesive under it, the toner contacts with the image carrier. Therefore, the width of the small gap is affected by the adhered toners to be overly large, and poor electrification of the image carrier is caused deteriorating the image quality.